1. Field of the Invention
The present invention relates to a semiconductor element and its manufacturing method, particularly to a functional semiconductor element such as a photovoltaic element and a thin film transistor and its manufacturing method.
2. Related Background Art
Microcrystalline silicon semiconductors have been presented in 1979 (S. USUI and M. KIKUCHI, xe2x80x9cPROPERTIES OF HEAVILY DOPED GD-Si WITH LOW RESISTIVITYxe2x80x9d, Journal of Non-Crystalline Solids, 34 (1979), pp. 1 to 11). This article has described that a low-resistivity microcrystalline silicon semiconductor doped with phosphorous was able to be deposited by a glow discharge method.
The same fact is described in A. MATSUDA, S. YAMASAKI et al., xe2x80x9cElectrical and Structural Properties of Phosphorous-Doped Glow-Discharge Si:F:H and Si:H Filmsxe2x80x9d, Japanese Journal of Applied Physics, Vol. 19, No. 6, JUNE, 1980, pp. L305 to L308.
Further, A. Matsuda, M. Matsumura et al., xe2x80x9cBoron Doping of Hydrogenated Silicon Thin Filmsxe2x80x9d, Japanese Journal of Applied Physics, Vol. 20, No. 3, MARCH, 1981, pp. L183 to L186 discusses the characteristics of a mixed-phase of boron-doped amorphous and microcrystalline silicon.
A. MATSUDA, T. YOSHIDA et al., xe2x80x9cStructural Study on Amorphous-Microcrystalline Mixed-Phase Si:H Filmsxe2x80x9d, Japanese Journal of Applied Physics, Vol. 20, No. 6, JUNE, 1981, pp. L439 to L442 discusses the structure of an amorphous and microcrystalline mixed-phase.
However, the possibility that such mixed layers of amorphous and microcrystalline silicon could be applied in semiconductor elements such as solar cells has been suggested, but there has been no actual application.
Solar cells using microcrystalline silicon semiconductors have been described in U.S. Pat. No. 4,600,801 xe2x80x9cFLUORINATED P-DOPED MICROCRYSTALLINE SILICON SEMICONDUCTOR ALLOY MATERIALxe2x80x9d, U.S. Pat. No. 4,609,771 xe2x80x9cTANDEM JUNCTION SOLAR CELL DEVICES INCORPORATING IMPROVED MICROCRYSTALLINE P-DOPED SEMICONDUCTOR ALLOY MATERIALxe2x80x9d, and U.S. Pat. No. 4,775,425 xe2x80x9cP- AND N-TYPE MICROCRYSTALLINE SEMICONDUCTOR ALLOY MATERIAL INCLUDING BAND GAP WIDENING ELEMENTS, DEVICES UTILIZING SAMExe2x80x9d. However, the microcrystalline silicon semiconductors described in these patents have been used in p-type or n-type semiconductor layers in solar cells of a pin structure using an amorphous i-type semiconductor layer.
Recently, articles on solar cells using microcrystalline silicon in an i-type semiconductor layer have been published. For example, there is xe2x80x9cON THE WAY TOWARDS HIGH EFFICIENCY THIN FILM SILICON SOLAR CELLS BY THE MICROMORPH CONCEPTxe2x80x9d, J. Meier, P. Torres et al., Mat. Res. Soc. Symp. Proc., Vol. 420, (1996) p. 3. However, as acknowledged by the authors of the article, the initial photoelectric conversion efficiency in a single structure solar cell using microcrystalline silicon in an i-type semiconductor layer is 7.7%, which is lower than that for solar cells with the same structure using amorphous silicon.
The present inventors have diligently inspected the reason why the conversion efficiency of solar cells using microcrystalline silicon semiconductors in an i-type semiconductor layer is lower than that of amorphous silicon solar cells with the same structure. The results make clear that the main cause lies in the interfaces between the n-type semiconductor layer or the p-type semiconductor layer with the i-type semiconductor layer. Specifically, the present inventors have discovered that there are many defect states in the vicinity of the interface of the n-type semiconductor layer with the i-type semiconductor layer as well as in the vicinity of the interface of the p-type semiconductor layer with the i-type semiconductor layer, which function as recombination centers. The existence of the recombination centers results in reduction in number and lowering in transportability of photo-excited free carriers in the i-type semiconductor layer. As a result, the open circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF) of the solar cell decline. Further, it is attributable to an increase in series resistance and a decline in the shunt resistance of the solar cell . As a result, the conversion efficiency of the solar cell declines.
When the inventors teamed up a transmission electron microscope with a secondary ion mass spectrometer and searched for the cause of the many defect states in the vicinity of the interfaces mentioned above, they discovered that the n-type semiconductor layer and the i-type semiconductor layer, or the p-type semiconductor layer and the i-type semiconductor layer were discontinuously stacked. Thus, they assumed that the reason why there were many defect states in the vicinity of the interfaces mentioned above was that the n-type semiconductor layer and the i-type semiconductor layer, or the p-type semiconductor layer and the i-type semiconductor layer were discontinuously stacked.
Further, when usual semiconductor elements are left to stand in the atmospheric environment, molecules in the air (water, oxygen, nitrogen, nitrogen oxides, sulfurous compounds, etc.) or the elements contained therein may sometimes diffuse into the semiconductor element to lower the characteristics of the semiconductor element. Similarly, when a semiconductor element such as a solar cell is encapsulated with another material (encapsulant), a chemical substance (acetic acid, etc.) contained in the encapsulant may sometimes diffuse into the semiconductor element to lower the characteristics of the semiconductor element. In particular, when each layer is stacked discontinuously at the semiconductor junction (junction of n-type semiconductor layer with i-type semiconductor layer, junction of p-type semiconductor layer with i-type semiconductor layer, etc.), the diffused substance will be trapped by the interface defects to lower the semiconductor characteristics.
When the inventors analyzed transmission electron microscope and X-ray diffraction data, it became clear that structural distortions were liable to be concentrated in the relatively large spaces between microcrystal grains, where there were many defects. These defects will reduce the trans-portability (mobility) of photo-excited free carriers and shorten the life time thereof to lower the characteristics of the semiconductor element.
The present invention aims to solve the above-mentioned problems and improve the photoelectric conversion efficiency of a photoelectric conversion element represented by a solar cell.
The present invention also aims to eradicate the discontinuity in the semiconductor junction portion to thereby provide a semiconductor element with superior semiconductor characteristics.
The present invention further aims to reduce the defects between microcrystal grains and to dissolve the discontinuity at the semiconductor junction portion to thereby provide a semiconductor element with superior semiconductor characteristics.
In addition, the present invention aims to improve the heat resisting properties and durability of a semiconductor element.
A first aspect of the present invention is directed to a semiconductor element comprising microcrystalline semiconductor, having a semiconductor junction in a microcrystal grain.
A second aspect of the present invention is directed to a semiconductor element comprising a semiconductor layer having first electric characteristics, a semiconductor layer having second electric characteristics, and a semiconductor layer having third electric characteristics stacked in the named order, wherein a microcrystal grain is present extending over at least a portion of the semiconductor layer having the first electric characteristics and at least a portion of the semiconductor layer having the second electric characteristics.
A third aspect of the present invention is directed to a method of manufacturing a semiconductor element, comprising the steps of:
forming a semiconductor layer having first electric characteristics on a substrate;
crystallizing the semiconductor layer having the first electric characteristics; and
growing a crystalline semiconductor layer having second electric characteristics on the crystallized semiconductor layer having the first electric characteristics, thereby growing a microcrystal grain so as to extend over the semiconductor layer having the first electric characteristics and the semiconductor layer having the second electric characteristics.
A fourth aspect of the present invention is directed to a method of manufacturing a semiconductor element, comprising the steps of:
forming a crystalline semiconductor layer having first electric characteristics on a substrate; and
growing a crystalline semiconductor layer having second electric characteristics on the semiconductor layer having the first electric characteristics, thereby growing a microcrystal grain so as to extend over the semiconductor layer having the first electric characteristics and the semiconductor layer having the second electric characteristics.
A fifth aspect of the present invention is directed to a method of manufacturing a semiconductor element, comprising the steps of:
forming a semiconductor layer having first electric characteristics on a substrate;
growing a semiconductor layer having second electric characteristics on the semiconductor layer having the first electric characteristics; and
effecting annealing to form a microcrystal grain so as to extend over the semiconductor layer having the first electric characteristics and the semiconductor layer having the second electric characteristics.
A sixth aspect of the present invention is directed to a method of manufacturing a semiconductor element, comprising the steps of:
forming a crystalline semiconductor layer on a substrate; and
ion-implanting a dopant into the semiconductor layer to form a semiconductor junction in a microcrystal grain of the semiconductor layer.
A seventh aspect of the present invention is directed to a semiconductor element comprising microcrystalline semiconductor, having a region where microcrystal grains with different grain diameters are present as a mixture.
An eighth aspect of the present invention is directed to a semiconductor element comprising a semiconductor layer having first electric characteristics, a semiconductor layer having second electric characteristics and a semiconductor layer having third electric characteristics stacked in the named order, wherein microcrystal grains with different grain diameters are present as a mixture in at least one of the semiconductor layers.
An ninth aspect of the present invention is directed to a method of manufacturing a semiconductor element, comprising the step of generating a plasma in a gas phase to decompose a source gas thus forming a semiconductor layer comprising microcrystals on a substrate, wherein an electric power to be applied to the plasma is periodically changed to form a semiconductor layer comprising microcrystal grains of different sizes as a mixture.
A tenth aspect of the present invention is directed to a method of manufacturing a semiconductor element, comprising the step of generating a plasma in a gas phase to decompose a source gas thus forming a semiconductor layer comprising microcrystals on a substrate, wherein a halogen-containing gas is added at regular intervals into the source gas to form a semiconductor layer comprising microcrystal grains of different sizes as a mixture.